This invention relates to the construction and method for making organosilane surface treated musical instrument strings. More particularly, it relates to strings having superior corrosion resistance and longevity during storage and end-use, and low stiffness for improved playability and tonal quality. The present invention is particularly adapted for use with all stringed instruments including classical guitar, steel string guitar, titanium string guitar, violin, cello, dulcimer, banjo, mandolin, bass, piano, harpsichord, etc.
It is well known that strings under tension will vibrate when plucked, struck, or bowed at a characteristic fundamental frequency f1, accompanied by a spectrum of n harmonic frequencies, all proportional to the tension and inversely proportional to the mass per unit length of the string (see, for example, Science And Music by Sir James Jeans, Dover Publications, Inc., New York, 1937, reprinted 1968). This relationship can be expressed for an ideal string of zero stiffness by equation 1:fn=(n/2 L)(T/m)1/2  [1]where L is the speaking length of the string, T is the tension, and m is the mass per unit length. Note that in addition to the fundamental tone, a geometric series of lower-amplitude overtone vibrations are produced at integer values of n>1. These overtones excite a complimentary ensemble of instrument resonance frequencies whose amplitudes are dependent on both the type of instrument, and on the physical properties of the instrument's component materials. In fact, the overtones and the resultant resonance vibrations that they excite are responsible for each instrument's unique tone, or timbre. In addition, because they are lower in amplitude than the fundamental tone, the overtones are the first vibrations to be perceptibly dampened by 1) frictional losses from string corrosion byproducts, and ironically 2) by mechanical losses from polymer coatings and covers that otherwise are designed to slow down the corrosion of musical instrument strings.
Low frequency musical instrument strings are often helically wound with mass loading materials that are susceptible to corrosion, including alloys of copper, steel and nickel, so that the unit mass can be controlled within some “window of tension” without having to increase the speaking length or the mass of the core wire. Otherwise, the speaking lengths for bass strings would be unrealistically long, and/or the diameter and mass would be too high, leading to high stiffness, reduced tonal quality, and difficulty with fingering during instrument play.
The “window of tension” is determined in part by the construction and design of the instrument, and specifically by the cumulative tension that can be sustained when a plural set of strings is tuned to pitch. Thus, if the tension is maintained at too high of a value, the instrument can be permanently damaged. If the tension is too low, then unwanted resonances and buzzing noises may occur. For example, the cumulative tension for strings on a “classical guitar” is typically between 75 and 100 pounds, whereas the cumulative tension on a steel-stringed acoustic guitar can be as high as 190 pounds.
The most commonly used materials for the cores of wound musical instrument strings include polymers such as synthetic nylon, natural “gut”; and steel (for example, music spring wire that is currently manufactured according to ASTM A228 specifications). Other cores include those made with aramide fibers as disclosed in U.S. Pat. No. 4,854,213 to Infield (1989); those made with composite cores of fibrous materials such as carbon, ceramic, or metal as disclosed in U.S. Pat. No. 5,578,775 to Ito (1996); and those made with alloys of titanium as disclosed in U.S. Pat. No. 6,348,646 to Parker et al. (2002). Each of theses patents is incorporated herein by reference in its entirety.
Although the use of metal windings has historically enabled designers to control mass per unit length and hence pitch, one inherent problem with wound strings is that certain windings and cores tend to corrode during both storage and end use. This leads to increasingly higher frictional losses and vibrational damping, with the upper harmonic frequencies being particularly affected. Gradually, the tonal qualities deteriorate and the strings lose their “liveliness” and “brilliance.” The problem may be partly related to stress relaxation from winding recoil, but it is also compounded by interfacial deterioration from corrosion at the core/winding interfaces, and from yielding of ductile interfacial materials such as tin or tin alloys.
Steel-core corrosion byproducts such as Fe2O3 are also weak oxides, and can easily spall, leading to mechanical losses and oxide particle contamination which can further dampen vibrations and negatively impact tonal quality. In addition, conventionally wound steel-core strings are often comprised of materials that are galvanically mismatched, and hence the propensity for corrosion is always present. Salt and moisture from human hand contact can create a type of salt-bridge that completes the potentially deleterious electrochemical couple between the winding and core. Collectively, these problems ultimately lead to what many musicians recognize as a “dead” string.
Many conventionally wound strings also have a limited shelf-life, and often require special packaging considerations and/or storage conditions to prevent corrosion, and to preserve their tonal characteristics prior to use. In some cases, strings that have been stored for long periods can become weakened from corrosion, and can break when attempts are made to tune the strings to pitch. In other cases, the otherwise “new” strings can exhibit the tonal characteristics of “dead” strings simply because they were stored too long before use.
Several prior art examples have attempted to address one or more of these issues through methods and constructions aimed at improving the longevity of wound strings. For example, U.S. Pat. Nos. 5,883,319; 5,907,113; and 6,528,709 to Hebestreit, et al. (1999, 2003) disclose wound and non-wound musical instrument strings that are covered with a porous polytetrafluoroethylene polymer over a portion of the speaking length, or over the entire speaking length of the string for the purpose of providing corrosion resistance, comfortable play, less finger noise, and longer life (these strings are commercially available from W. L. Gore & Associates, Inc.). U.S. Pat. No. 5,801,319 to Hebestreit, et al. (1998) teaches that the mass of said coverings represent approximately 3% of the mass of the entire covered string. Thus, since the cover traverses the speaking length of the string, its mass therefore modulates the tonal and dampening characteristics accordingly.
U.S. Pat. No. 2,892,374 to Ralls (1959) discloses a conditioning process where a musical instrument string with a metallic winding wrapped about a gut core is treated by soaking the strings in a polymer lacquer solution to coat the core and to fill the interstitial spaces between core and windings along the entire speaking length of the string. The purpose is to prevent shrinkage of the gut core and to prevent loosening of windings during end use, both of which lengthen string life. Similarly, since the polymer traverses the entire speaking length, tonal quality, stiffness, and playability are affected accordingly.
U.S. Pat. No. 2,049,769 to Gray (1936) discloses a string constructed with a polymeric, varnish-reinforced fabric that encircles a straight or kinked metal core along its entire length to form a unitary string body. Metal windings can be incorporated either between the fabric and core, or they can be wrapped around the unitary composite core. This string owes its properties to its composite nature, where the fabric is incorporated to carry a portion of the tensile load in concert with a steel core wire. The polymer and fabric traverse the entire speaking length, so tonal qualities, dampening, stiffness, and playability are all affected.
U.S. Pat. No. 4,539,228 to Lazarus (1985) discloses a method for treating wound musical instrument strings to reduce the “break-in period” and to extend useful life by filling microscopic pores, cavities, and interstitial spaces of wound strings with dry lubricant particles, moisture displacement agents, and a corrosion inhibitor, where the proposed corrosion inhibitor is broadly described as being comprised of a parafinic oil and a wetting agent. Although the use of a corrosion inhibitor is generally discussed, no insight is provided with respect to the effects of molecular structure on chemisorption, passivation, and corrosion inhibition; nor is insight provided with respect to the effects of galvanically mismatched materials on the efficiency of corrosion inhibition. Thus, although the concept of corrosion inhibition is recognized to be of value, no insight is provided with respect to the molecules and surface concentrations that could be appropriate for protecting metals that are subject to the galvanic conditions that are specific to a musical instrument string's metallic construction and composition. Of equal importance, the dried particulates are chosen so as to fill the interstitial spaces between the windings, and to traverse the speaking length of the string. These non-bonded particulates are therefore expected to modulate the tonal and dampening characteristics accordingly.
U.S. Pat. No. 6,348,646 to Parker et al. (2002) discloses strings comprised of titanium alloy cores (commercially available from Rohrbacher Technologies at www.rohrtech.com), wherein both wound and non-wound strings have superior corrosion resistance and low stiffness for improved playability. Unlike traditional steel-core strings, the titanium core string is comprised of a relatively cathodic metal core (cathodic because titanium metal is passivated with a thin oxide layer), and a second metal winding wire, where the difference in galvanic potential between the two metals (as measured by the difference in galvanic potential with respect to a saturated calomel electrode in seawater) is as close to zero as possible. The best corrosion resistance is achieved when the core and winding wires are similar in galvanic potential, and when the core is the more cathodic member of the coupled pair. Consequently, superior corrosion resistance is achieved with nickel wound titanium alloy core strings, where the galvanic mismatch is essentially nil. Excellent corrosion resistance is also achieved with copper-alloys wound about titanium-alloy cores, particularly when the strings are surface treated with an azole compound.
However, copper alloys such as brass and phosphor bronze still have the propensity to corrode, even when the cores of the strings are comprised of titanium alloys, and even when the strings are surface treated with azole compounds. Thus, although azole surface-treated titanium-alloy strings provide a substantial improvement over the prior art, there still exists a need for methods by which to further improve the corrosion resistance and longevity of all musical instrument strings that are wound with copper alloys.
Coatings, covers, lacquers and the like have been used in attempts to arrest the corrosion process. Unfortunately, such coatings also reduce the brightness of the strings during use. Again, the perceived brightness of a string arises from its ability to excite the resonance vibrations of a musical instrument. Anything that interferes with these vibrations will deteriorate sound quality. Thus, corrosion byproducts, contamination from finger contact, and even coatings that are designed to help prevent corrosion can all contribute to the dampening of string vibrations.
It is generally known that corrosion of the anodic component in a galvanic couple is accelerated as the ratio of the surface area of the cathodic metal to the anodic metal increases (Metals Handbook Desk Edition, second edition, J. R. Davis-Editor, ASM International, Materials Park, Ohio, 1998). In the case of conventionally wound steel core strings, the steel core is typically the more anodic of the coupled pair, and it also has the least amount of exposed surface area. Even worse, the iron oxides that form at the anode are mechanically weak oxides, which easily spall, leading ultimately to shearing motions and contamination at multiple interfaces, and vibrational dampening in the form of frictional heat dissipation. In order to minimize corrosion, it is desirable to either construct the string with electrochemically equivalent materials, or if some degree of galvanic coupling is inevitable, to design the string by minimizing the surface area of the cathodic member. This condition is satisfied when the core member is comprised of titanium or a titanium alloy. In addition, regardless of which member is more anodic or cathodic, it is generally desirable to passivate or protect the interfaces of the most corrosion-prone component members, especially when a galvanic mismatch exists.
The oxide layer that forms on the surface of metals in air is a protective layer that itself can inhibit the corrosion process. It is known to those skilled in the art of surface chemistry and corrosion inhibition that certain metal oxide layers are durable and resistant to hydrolysis, permeation, and mechanical wear. Metals that form these types of oxide layers are by nature extremely corrosion resistant (titanium and its alloys fall into this category). On the other hand, if the protective oxide layer that forms on a metal is mechanically weak, and/or if it is prone to hydrolysis and moisture/oxygen permeation, then the protective layer may spall and expose fresh metal surfaces which in turn are prone to continued oxidation (iron and, to a lesser degree, copper alloys fall into this category). This cycle, otherwise known as corrosion, continues at a rate that depends on many environmental factors, as well as on the inherent properties of the metal.
One generally accepted method of protecting a metal from corrosion is to use a corrosion inhibitor to chemically stabilize the protective oxide layer on the metal surface. A good corrosion inhibitor generally chemisorbs onto the metal oxide surface to form a stable chemical bond (the bond can be covalent or ionic in character). The formation of a bond can often be observed spectroscopically through the use of surface analytical techniques that are known to those skilled in the art (i.e., FTIR, ESCA, etc.). Some of the best corrosion inhibitors have been found to form surface complexes that are resistant to hydrolysis, oxygen permeation, water permeation, and dissolution. This is the generally accepted mechanism by which azole compounds are thought to protect the surfaces of copper alloys. In fact, spectroscopic evidence has shown that azole compounds form protective organo-cupric polymeric complexes that resist dissolution and moisture permeation (see for example, J. B. Cotton and I. R. Scholes, Brit. Corrosion J, 2, 1-5, 1967; and J. C. Rubim, Chem. Phys. Lett., 167, 209-214, 1990).
The protective polymeric complexes are formed when certain azole compounds are either deposited from solution onto the surfaces of copper alloys, or when the copper alloys are stored in contact with azole treated paper packages. Importantly, this type of treated paper is often used to cover and protect spools of copper alloy wires that are used to construct musical instrument strings. In addition, azole impregnated paper is often used to make the familiar envelopes that are used to store many types of musical instrument strings.
Although chemisorption is recognized as an important factor in corrosion inhibition, it can also be detrimental. In fact, some chemisorbed compounds actually accelerate the corrosion of metals. For example, this can happen when the resultant chemisorbed complex is more water-soluble and is less resistant to permeation than the original oxide layer. This is why certain acids and bases can have a deleterious effect on metal corrosion. For example, certain primary amine compounds, including ammonia, readily attack copper alloys. Thus, chemisorption alone does not guarantee that a compound will inhibit corrosion.
Many steel cores for wound guitar strings are typically surface treated with malleable metals such as tin, tin alloys, gold, or silver for the purpose of decreasing the rate of corrosion, and for helping to maintain initial winding tightness. For example, U.S. Pat. No. 4,063,674 to Stone and Falcone (1977) discloses a method of manufacture whereby a wound string assembly is heated at an elevated temperature for various amounts of time to produce a string where windings are more evenly spaced. The coefficient of thermal expansion of the core is less than that of the winding, and the core is coated with a material having a melting point lower than the heat treatment temperature. The invention discloses a tin coating that upon heating, can be used to form a metallurgical bond between winding and core.
It is generally known that surface coatings such as tin can reduce the galvanic couple between steel and other metallic materials, but corrosion is not entirely prevented (see for example McKay, R. J. and Worthington, R., Corrosion Resistance of Metals and Alloys, American Chemical Society Monograph Series, Reinhold Publishing Corporation, New York, 1936). The malleability of tin can enable it to yield and partially encase the winding during processing to help maintain initial tightness. However, this same attribute can also be a long term detriment since the ductility of tin renders it susceptible to yielding under the recoil stress of the windings, a problem which is further aggravated by corrosion since bi-products may further weaken the material near the chemically dissimilar interfaces. Thus, short term durability and ultimate interfacial failure are simultaneously and paradoxically inherent to the structural design of many conventional metal wound steel core strings.
In cases where polymers or lacquers have been used to either slow corrosion or to maintain winding tightness, they traverse either a portion of, or the entire speaking length of the string, and thus they influence the tonal and dampening characteristics of the string.
Alternatively, lower density polymeric strings such as gut and nylon are not susceptible to corrosion, and are used either alone or as the cores for metallic or polymeric wound strings. However, the metallic windings that are sometimes used in combination with these cores are still susceptible to corrosion.
Accordingly, it would be desirable and advantageous to develop a surface treated string construction wherein the use of sound dampening polymeric materials, covers, lacquers and particulates is minimized or eliminated over the speaking length of the string. In addition, it would be further advantageous to choose a surface treatment that chemisorbs to form a stable, moisture and heat resistant bond that resists dissolution, wear, and dimensional change during manufacturing, during storage, during tuning, and during end-use. Furthermore, in the event that a galvanic couple between the contact metal surfaces is inevitable, then it would be desirable for the lowest surface area member (the core) to be the more cathodic member of the surface treated construction.
Accordingly, a primary object of the present invention is to provide a surface treated wound metallic musical instrument string with combined attributes of high corrosion resistance, and durability.
Another object is to provide a low-stiffness, surface treated, corrosion resistant musical instrument string having the associated benefits of low stiffness, including ease of play, and better tonal qualities.
Another object is to provide a musical instrument string with the benefits of improved corrosion resistance including longer shelf life before use, and longer life during end use.
Yet another object of the present invention is to provide a method for manufacturing the strings of this invention whereby the resultant surface treated string provides optimum corrosion protection both during manufacture and in end use.
Still another object is to provide a method for maintaining a high degree of corrosion resistance without the use of dampening polymeric covers within the speaking length of the string.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.